The invention relates generally to optical frequency routers. More particularly, the invention relates to a waveguide optical frequency router having an array of distributed Bragg deflectors.
Many emerging applications such as electronic warfare and telecommunications rely on the ability to route optical signals to particular destinations depending upon the frequency of the optical beam. For example, dense wavelength division multiplexing (DWDM) requirements are such that optical carriers differing in frequency by 50 GHz must be separated. Even more stringent requirements exist for photonic radio frequency (RF) spectrum analyzers, where it is desirable to sort an optical carrier that is modulated with an RF signal into 1 GHz wide or smaller frequency channels.
One approach to wavelength division multiplexing involves the use of bulk diffraction gratings. Under this approach, an input signal containing several optical wavelengths will be directed to a collimating lens, which projects the input signal on one or more diffraction gratings. A diffraction grating comprises a plurality of parallel lines or grooves spaced extremely closely together. Light incident on the lines produces a rainbow spectrum with each wavelength spread through a different angle as a result of diffraction. Additional lenses can be used to focus the diffracted light onto photo detectors or optical fibers (depending upon the application). One particular shortcoming of the bulk diffraction grating approach is that it results in relatively large devices that are sensitive to environmental influences. In fact, in order to achieve the 1 GHz resolution requirement of certain applications, a device approximately 2xe2x80x2X3xe2x80x2X2xe2x80x2 would be required.
Another approach to wavelength division multiplexing involves the use of the arrayed waveguide grating (AWG). The AWG approach typically involves fabricating several hundred channel waveguides onto a substrate. By precisely controlling the length of each channel waveguide, the AWG is able to distribute the optical energy according to the wavelength between various output channel waveguides formed on the substrate. It is important to note that in order to achieve an optical frequency resolution of 1 GHZ, the length of the channel waveguides must be controlled to within a fraction of an optical wavelength. The AWG is therefore complex and quite difficult to manufacture.
Furthermore, the channel response characteristics for both of the above approaches are difficult (and in some cases impossible) to tailor without incurring excess complexity and additional optical loss. It is therefore desirable to provide a waveguide optical frequency router that is compact, monolithic, and can achieve optical frequency resolutions in the range of 1 GHz. It is also desirable to provide a waveguide optical frequency router whose frequency response can be readily tailored to suit the needs of a variety of applications, and can easily be manufactured.
The above and other objectives are provided by a waveguide optical frequency router in accordance with the present invention. The router has a transmit 2-dimensional optical waveguide core region formed within a planar optical waveguide slab region. A plurality of receive core regions are also formed within the slab region. The router further includes an array of Bragg gratings formed within the core regions for coupling optical energy between the transmit core region and the receive core regions via the slab region. Together the slab region, core region, and Bragg grating constitute a distributed Bragg deflector (DBD). The array of DBDs distributes the optical energy between the receive core regions based on wavelength and propagation angle. The array of DBDs therefore provide compactness, and high optical frequency resolution.
Further in accordance with the present invention, an array of DBDs is provided. The array includes a transmit DBD formed within a transmit core region. The transmit DBD diffracts optical energy having a first wavelength into a slab region at a first angle, and diffracts optical energy having a second wavelength into the slab region at a second angle and similarly for a plurality of optical wavelengths. A first receive DBD diffracts only optical energy of the first wavelength propagating at the first angle into the first receive core region. The first receive DBD also passes all other optical energy through the first receive core region and into the slab region. In a preferred embodiment, a second receive DBD is formed where the second receive DBD diffracts optical energy of the second wavelength propagating at the second angle into the second receive core region, and similarly for a plurality of receive DBDs.
In another aspect of the invention, a method for fabricating an array of DBDs is provided. The method includes the step of forming a transmit DBD comprising a transmit core region and a Bragg grating formed in a slab region. The transmit DBD diffracts optical energy having a first wavelength into a slab region at a first angle and diffracts optical energy having a second wavelength into the slab region at a second angle and similarly for a plurality of wavelengths. A first receive DBD is formed where the first receive DBD diffracts only optical energy of the first wavelength propagating at the first angle into the first receive core region. The method further provides for forming a second receive DBD, where the second receive DBD diffracts only optical energy of the second wavelength propagating at the second angle into the second receive core region and similarly for a plurality of receive DBDs. In a highly preferred embodiment, the diffraction gratings for the receive DBDs are formed by holographically projecting a predetermined fringe pattern onto the receive core regions, and generating gratings having periods and angles in accordance with the predetermined fringe pattern.